Century-old forest study reveals troubling decline in tree diversity

Across the eastern United States, tree communities are shifting toward fewer species in a process known as homogenization. A long-term case study now shows how quickly a decline in tree diversity can unfold inside a single Midwestern forest.

The 24-acre Trelease Woods, owned by the University of Illinois since 1917, has been surveyed again and again for almost a century.

Tracking a century of forest diversity

The team mined plot maps, punch-card data, and modern laser measurements to follow every tree wider than 2.5 inches recorded since 1922. During that period, species richness slipped from 23 to 20 as sugar maple and Ohio buckeye seized much of the growing space.

Trelease now forms part of the ForestGEO network, which monitors roughly seven million trees representing nearly 13,000 species worldwide. Linking the local census to that global database allows scientists to compare small-scale stories with continental patterns.

Tree diversity fell 22 percent overall, and the steepest drop showed up in saplings shorter than five inches across. That decline signals a forest less able to roll with surprise pests, storms, or heat waves.

Average tree diameter shrank by one-fifth as tiny saplings crowded below a still-expanding maple canopy. Yet the three largest trunks on the plot, two bur oaks and a black walnut, grew even larger – showing that veteran giants can coexist with maple expansion.

Pests and declining tree diversity

The invasive emerald ash borer kills more than 99 percent of untreated ash trees and could cost U.S. cities an estimated 900 million dollars over the next three decades. Dutch elm disease, driven by the fungus Ophiostoma novo-ulmi, has already erased most mature American elms in many states.

“Dutch elm disease and the emerald ash borer have significantly decreased the abundance of elm and ash trees in Trelease Woods and across North America,” said Jennifer Fraterrigo, who helped lead the analysis.

When the insects and fungi hollow out elms and ashes, they open holes in the canopy. Shade-loving seedlings waiting below pounce on the new light before slower oaks or hickories can respond.

Ash logs that once shaded summer research crews now lie riddled with larval galleries, their distinct D-shaped exit holes signaling where beetles fled.

Because ash and elm filled different ecological niches – elm trunks provided shelter for cavity-nesting birds, while ash trees rapidly recycled nutrients – their simultaneous loss intensifies ecosystem disruption.

Fires and forest diversity

Early last century, federal and state policies pushed to stamp out most woodland fires, and that retreat of flames helped tip forests toward shade-tolerant, mesic species.

Ecologists call the feedback loop mesophication, because the stand grows cooler, wetter, and ever friendlier to maples and other moisture-loving trees.

At Trelease, fire scars are absent in modern cross-sections, and the canopy has steadily darkened. That shift leaves oak seedlings gasping for sun while sugar maples prosper under their own shade.

Sapling counts confirm the change: maple stems now outnumber oaks almost ten to one in the smallest size class. Without periodic burns, managers say, the imbalance will only grow.

Researchers elsewhere have shown that just a few carefully timed burns can reverse mesophication, but only if enough oak seedlings remain in the bank. The Trelease dataset hints that the window for that turnaround is closing as maple dominance widens.

Deer change the understory

Soaring numbers of white-tailed deer compound the problem by selectively browsing palatable seedlings and suppressing regeneration. Browsed stems stay short, letting shade widen across the forest floor.

Hackberry, basswood, and ash suffer the most nibbles here, while less tasty pawpaw spreads in their place. Those dietary choices accelerate the compositional skid already set in motion by pests and fire exclusion.

Even sugar maple, a deer favorite, persists because enough seedlings dodge grazing to replenish each gap. The result is a patchwork where a few hardy winners dominate every height layer.

Camera traps inside the woodlot now record up to eight deer per square mile, double early-1990s estimates kept by state biologists. Such densities far exceed the one to three deer per square mile many ecologists recommend for oak regeneration.

Why tree species diversity matters

Losing species diversity strips away the ecological insurance that forests rely on when drought, disease, or storms strike. Fraterrigo warns that if one species swells to 70 percent of stems, a single new pest could trigger what she calls functional collapse.

Economic stakes are high as well, because mixed stands store more carbon and support more wildlife than single-species blocks. Long-term monitoring like the Trelease census offers early warning before those services vanish.

Tree variety underpins a mosaic of flowering times, leaf chemistries, and root depths that spreads risk the way a smart investor spreads capital. Homogenization erases that portfolio, leaving the system riding on a single economic bet.

Broader implications of the study

Researchers suggest thinning dense maple thickets, reintroducing low-intensity burns, and culling deer to rebalance light and browsing pressure. Planting diverse native saplings in canopy gaps can also jump-start forest recovery.

Lessons from Trelease now inform experiments across the ForestGEO network as scientists test which combinations of fire, harvest, and wildlife control restore forest resilience. Managers elsewhere may adapt those findings to keep their own woods from sliding into maple monocultures.

Some managers have begun injecting systemic insecticides into lingering ash saplings to preserve genetic stock until biocontrol agents catch up. Others experiment with protective cages around oak seedlings to give them a head start against hungry deer.

The study is published in the journal Forest Ecology and Management.

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